1. Field of the Invention
This invention relates generally to magnetic random access memory (MRAM) and more particularly to MRAM with “toggle” memory cells.
2. Description of the Related Art
MRAM with magnetic tunnel junction (MTJ) memory cells has been proposed for nonvolatile memory, as described in U.S. Pat. No. 5,640,343 and by Reohr et al., “Memories of Tomorrow”, IEEE CIRCUITS & DEVICES MAGAZINE, September 2002, pp. 17–27. In these devices the MTJs are arranged as an array in a single layer (the X-Y plane) on a semiconductor substrate. In one type of architecture, called a 1T1MTJ MRAM (one transistor and one MTJ), each MTJ is located between a bit line and a transistor, with the word lines located beneath the MTJs. In another type of architecture, called a cross-point (XPC) MRAM, the MTJs are located directly between the bit and word lines.
In both MRAM architectures, a selected MTJ cell is programmed or “written”,i.e., its magnetic state or +/−X magnetization direction is switched, by write currents passing in X and Y directions through the bit and word lines located above and below the selected MTJ. The write currents generate orthogonal magnetic fields in the X and Y directions that switch the magnetization direction of the selected MTJ. The typical writing scheme is a “half-select” scheme, where each of the bit and word lines generates half the required write field for switching the selected MTJ cell. However, the energized word and bit lines reduce the magnetic reversal energy barrier in the other cells along their respective word and bit lines. This makes these “half-selected” cells more susceptible to having their magnetic states switched when the selected cell is written.
An MRAM with a MTJ cell structure and switching mechanism that does not suffer from the half-select problem of the conventional MRAM has been proposed by Motorola. This “Savtchenko” cell structure and switching mechanism, named for its late inventor, is described in U.S. Pat. No. 6,545,906 B1 and M. Durlam et al., “A 0.18 μm 4 Mb Toggling MRAM”, IEDM Technical Digest 2003, Session 34, paper #6. In this type of MRAM, the MTJ cell's ferromagnetic free layer is a synthetic antiferromagnet (SAF), i.e., a multilayer of two ferromagnetic sublayers of nearly identical magnetic moment, separated by an antiferromagnetic coupling layer that maintains an antiparallel alignment of the moments of the two sublayers. An SAF free layer in a spin-valve magnetoresistive sensor is described in U.S. Pat. No. 5,408,377, and an MTJ memory cell with SAF free and pinned layers is described in U.S. Pat. No. 5,966,012. The Savtchenko type of MRAM uses two orthogonal writing or programming lines, but with the MTJ cell's axis aligned 45 degrees to each of the lines. The SAF free layer responds to applied magnetic fields differently than a conventional single ferromagnetic free layer. Writing occurs by a process called “toggle” writing in which a two-phase programming pulse sequence incrementally rotates the SAF free layer moment or magnetization direction 180 degrees, so the MRAM is sometimes called a “toggling” MRAM and the memory cell a “toggle” cell. Because of the cell's 45 degree angle to the programming lines and its field response, the field from a single programming line cannot switch the magnetization of a half-selected cell, which results in an MRAM with enhanced cell selectivity.
The toggling MRAM is a single-memory-layer MRAM, i.e., all the toggle memory cells lie in substantially the same horizontal plane (the X-Y plane) on the substrate, so the storage density is necessarily limited. A toggling MRAM with specially-shaped toggle memory cells having multiple easy axes of magnetization and capable of more than two magnetic states has been described in published patent application U.S. 20040012994A1, but this MRAM is also a single-memory-layer MRAM.
What is needed is a toggling MRAM that does not require all the toggle memory cells to lie in the same plane, so that the storage density can be increased vertically, i.e., in the Z direction from the substrate.